pool: Remove caching behaviour of file meta backend

Revert 8290f2f878a818a1398bea7c415a5b1274ec47eb

Tigran reported that the pool performs badly with the
simple caching behaviour added in the above patch.

Target: trunk
Request: 2.6
Require-notes: yes
Require-book: no
Acked-by: Tigran Mkrtchyan <tigran.mkrtchyan@desy.de>
Patch: http://rb.dcache.org/r/5608/
Merge branch 'master' of https://github.com/dCache/dcache